Heavy metal and sulfur transport during subcritical and supercritical hydrothermal alteration of basalt: Influence of fluid pressure and basalt composition and crystallinity

Crystalline basalt, diabase and basalt glass have been reacted with a Na-Ca-K-Cl fluid of seawater ionic strength at 350–425°C, 375–400 bars pressure and fluid/rock mass ratios of 0.5–1.0, to assess the role of temperature, basalt/diabase chemistry and texture on heavy metal and sulfur mobility during hydrothermal alteration.Alteration of basalt/diabase is characterized by cation fixation and hydrolysis reactions which show increased reaction progress with increasing temperature at constant pressure. Correspondingly, pH in a series of 400 bar experiments ranges from 4.8 to 2.7 at 350 and 425°C, respectively and is typically lower for alteration of a SiO₂-rich crystalline basalt than for other rock types, due, in part, to relatively high SiO₂ concentrations in solution. High SiO₂ concentrations stabilize hydrous Na- and Ca-rich alteration phases, causing pH to decrease according to reactions such as: 3.0 CaAl₂Si₂O₈ + 1.0 Ca⁺⁺ + 2.0 H₂O = 2.0 Ca₂Al₃Si₃O₁₂(OH) + 2.0 H⁺Phases experimentally produced include: mixed layer chlorite/smectite, Ca-rich amphibole and clinozoisite. Clinozoisite was identified as a replacement product of plagioclase from diabase-solution interaction experiments.In direct response to H⁺ production, dissolved Fe, Mn and H₂S concentrations increase dramatically. For early-stage reaction, H₂S typically exceeds Fe and Mn. However, at 425°C and after long-term reaction at 400°C, H₂S is lost from solution, apparently in response to pyrite replacement of oxide and silicate phases.Pyrrhotite formed at temperatures ≤ 375°C, whereas magnetite was identified in all run products, except from basalt glass alteration.Cu and Zn concentrations in solution are not simple functions of pH. These metals achieve greatest solubility in fluids from experiments at 375–400°C, except when basalt glass is used as a reactant. The relatively low concentrations of these species in solution during basalt glass reaction may be due to adsorption by fine grained alteration phases.